The magnetic sensors (digital and analog) of linear or angular position have many advantages:                no mechanical contact with the mobile portion, and hence no wear,        insensitivity to dirt,        reduced production cost,        long service life.Magnetic sensors used for determining the position and/or the velocity of a ferromagnetic piece (piece exhibiting teeth on its periphery) are typically used in the automobile industry in order to know the position/velocity of the camshaft with the purpose of controlling the electronic ignition of the motor.        
Most contactless magnetic digital position sensors use cylindrical-shaped permanent magnets axially-magnetized associated to several magnetically sensitive members (differential probe) with the aim of eliminating the average magnetic flux density detected by the probes. In fact, given the shape of the magnet, the induction detected by a probe is very high and hence very sensitive to the variations of the magnet magnetic properties along with temperature variations. The calculation of the induction difference between the two magnetically sensitive members makes it possible to cancel the average magnetic flux density but on the other hand it becomes difficult to know the position of the member to detect at the starting of the system. The use of several magnetically sensitive members also induces a more important cost and a higher global volume of the sensor.
There also exists systems which use a substantially cylindrical permanent magnet, comprising a cavity, associated to one single magnetically sensitive member. The magnet comprising a cavity makes it possible to reduce the average magnetic flux density at the magnetically sensitive member and hence the use of one single magnetically sensitive member. These systems also make it possible to know the position of the member to detect as soon as the sensor is started-up.
The current trend for this type of sensor is to have even smaller sensors without however losing the level of performance. The two types of systems described above do not allow for an appreciable reduction in size without compromising performance. Moreover, the distance between the member to detect (target) and the sensor is also more important and it is necessary to guarantee performance, this can only be done by an increase in the variation of the magnetic induction at the magnetically sensitive member according to the target position.
It is known, in the state of the art, patent FR2724722 of the applicant and U.S. Pat. No. 6,043,646 which describe a digital position/velocity sensor with a substantially cylindrical permanent magnet using one single magnetically sensitive member. The systems described make it possible to obtain at the magnetically sensitive member a average magnetic flux density very close to 0 G, however a decrease in size of the magnet leads to a decrease in the sensor sensitivity and hence a decrease in performance.
There is also known U.S. Pat. No. 5,781,005 describing a digital position sensor used for detecting a ferromagnetic piece which exhibits a plurality of teeth. This sensor uses two permanent magnets magnetized in a same direction and stuck to a ferromagnetic plate. A Hall effect probe placed above this assembly, between the target and the magnet, measures the variation of magnetic induction in a direction parallel to the magnetization. The magnetically sensitive member of the probe being placed in proximity to the target. With such a system, it is difficult to have an average magnetic flux density close to 0 G and a decrease in the external diameter leads to a decrease in performance.
There is also known in the state of the art, patent FR2845469 of the applicant, which describes a linear or rotary position analog sensor. This analog sensor measures the position of a moving ferromagnetic piece (linear or rotary) by means of the induction variation generated by the variation of reluctance between a ferromagnetic piece and a permanent magnet, the induction variation being measured by a Hall effect probe. The downfall of this system is that the decrease in size of the sensor which comprises the magnet and the probe leads to a decrease in performance and a difficulty to work with an average magnetic flux density at the Hall effect probe close to 0 Gauss.